Method of determining the risk of ice deposition due to precipitation and apparatus for exercising the method

ABSTRACT

The invention relates to a method and an apparatus for determining the risk of ice deposition due to precipitation. According to the invention the measurements of precipitation known so far are combined with measurements of the actual amount of ice deposited from the precipitation. Said measurements are combined in a combination unit that is able to receive further parameter information, eg the amount and type of anti-icing liquid. Compared to previously a considerably more reliable determination of the risk of ice deposition is accomplished, eg on the wing of an aeroplane applied with anti-icing liquid, and so is reliable determination of the holdover time, HOT, during which one can be sure that the aeroplane is free from ice in the current weather conditions.

The invention relates to a method of determining the risk of icedeposition due to precipitation. The invention is particularly, but notexclusively intended for use in connection with determination of therisk of icing in connection with air traffic.

When there is a risk of ice formation, the air traffic uses anti-icingliquids of various types and concentrations, and the problem is toestimate for how long the anti-icing liquid will stay effective duringthe existing weather conditions. This is referred to as ‘holdover time’;in the following designated ‘HOT’.

The international airline organisations publish tables that indicate atime interval for holdover time for some anti-icing liquids and forquite a small number of concentrations thereof. These tables, the onlytool available at present, are associated with two major factors ofuncertainty. Firstly the time intervals listed in the tables are givenwith large margins, eg a minimum of 30 and a maximum of 60 minutes, andsecondly the tables can be used only if it is possible to correctlyestimate the precipitation, the tables being divided in accordance withtypes of precipitation, such as eg snow or super-cooled water. The finalresponsibility for the estimation lies with the pilot, ie that fromwithin the cockpit, frequently in weather conditions such as followingwinds and through a heavily heated slanted window, he is to estimate thetype of precipitation and then take a stand on the minimum and maximumtime intervals given in the table.

The very varying types of precipitation that typically occur within thetemperature range of from 5° C. and 8° C. above freezing point are thecause of the longest delays in the airports and often the planes mustwait long to obtain permission to take off, while the anti-icing liquidis consumed more quickly or slowly in response to the precipitationconditions.

To date, 141 aircraft accidents have been ascribed to ice accumulationwith an ensuing death toll of 1200.

It is the object of the invention to provide a method that enablesdetermination of HOT, wherein the determination is based on actualmeasurements rather than the subjective estimations resorted to so farin the prior art.

This object is accomplished by the method exercised in accordance withthe characterising part of claim 1. As it is, a determination of thetype of precipitation or the equivalent amount of liquid cannot be takento express how much ice will be formed, since—by the known methods—it isnot possible to distinguish between super-cooled water and ordinarywater. This uncertainty is the greatest precisely within the temperaturerange where the risk of icing is the largest, viz around 0° C.,

By combining the measurements in accordance with the characterising partof claim 1, a complete and objective measurement is accomplished of theconditions that are significant to the estimation of the risk of iceformation when anti-icing liquid is used. The combination and itssignificance are explained in further detail in the context of FIGS. 2and 3.

The two measurements that are combined according to the invention caneach be obtained separately by techniques that are already known andthat can be performed by use of separate apparatuses or by means of acombined apparatus. For instance, the equivalent amount of liquid can bedetermined by means of the technique taught in U.S. Pat. No. 5,434,778.

In accordance with a preferred embodiment the actual content of ice inthe precipitation is determined by means of a measurement of the actualice formation, eg by means of the technique taught in WO 00/54078, seeclaim 6.

In accordance with one embodiment the temperature of the surface elementis caused to be essentially the temperature of atmospheric air, butalternatively the temperature of the surface element can be controlledto have another predetermined temperature. In this context, parameterssuch as the temperature of the fuel in the wing or that of thesprayed-on anti-icing liquid may be of relevance.

By the apparatus taught in WO 00/54078, a number of surface elements arerotated at a rate that is to ensure, on the one hand, that the ice isdeposited and, on the other, that the majority of water drops are flungoff. By exercising the method as recited in claim 10, it is ensured thatthe slow rotation does not reduce the actual ice formation, and the highrate of rotation ensures that no water remains on the rotor before theamount of ice deposited is weighed. The amount of ice can also bedetermined in other ways than weighing.

Moreover, it is expedient to perform further measurements, eg of thekind featured in claims 11 through 14.

By combining the reliable measurement of the risk of ice formation withknowledge of the type and concentration of the applied anti-icing liquidit is possible to achieve a more reliable estimate of the holdover time,HOT, to be expected for keeping the fly wings free from ice in the givenweather conditions. By the invention it is enabled that HOT can be givenwith a very reduced margin of insecurity compared to the prior art, seethe explanation in the context of FIGS. 2 and 3.

However, it often applies—in particular within the field of airtraffic—that a rather conservative approach is employed which willundoubtedly involve that some time will elapse before the pilots getused to having access to a well-defined holdover time. Undoubtedly, thewell known tables will be consulted for some time yet, of which one ofthe elements of insecurity was to determine the type of precipitation.By exercising the invention as taught in claim 17, the objectivedetermination that results from the invention can be used to give areliable indication of the actual composition of the precipitation.

Then the pilots can feel safe in, initially, verifying that the holdovertime according to the invention is within the maximum intervals given inthe tables and, subsequently, in complete confidence use the holdovertime according to the invention as a reliable, well-defined limit.

Safety being, of course, the top priority; there remains also the aspectthat the anti-icing liquid is expensive and that it is waste of moneyand associated with unnecessary pollution to apply more anti-icingliquid than needed to obtain safe flying. By exercising the invention asrecited in claim 15 it is possible to determine the smallest requisiteconcentration of the anti-icing liquid to be applied to accomplish adesired holdover time.

Apart from the above advantages, the invention provides options thatpresent completely new perspectives. By combining measurement equipmentfor determining the amount of precipitation and combinations withmeasurement equipment for measuring the amount of ice actuallydeposited, it is now an option to make a self-learning expert system asrecited in claim 19. According to the invention a holdover time isaccomplished that is far more reliable than the one used so far, basedon measurements, though, of actual weather conditions that applied fiveor ten minutes ago at most. The known tables are based on empiricalconditions that can be registered in a calculation mode with someparameters being automatically adjustable by comparison of thecalculated deposited amount of ice of the calculation model to theamount of ice actually measured. Thereby the risk of ice can be dulypredicted. By connecting computers in various airports to each other,and by inputting meteorological data the model can be expanded toprovide, based on meteorological data, an estimate of the risk of ice atother airports, and this estimate can be compared to the currentlymeasured ice accumulation at these airports, following which acalculation model can be dynamically optimised.

The invention also relates to an apparatus for exercising the methodaccording to claim 1. The apparatus is characterised by theconfiguration recited in claim 21.

Preferably the apparatus also contains a data storage with empiricalinformation on holdover time so as to provide a considerably morereliable determination of the actual holdover time, see claim 22.

The apparatus may also feature a computer with a mathematical model forestimating eg holdover time, wherein the model comprises a number ofadjustable parameters. By comparing the estimated results to the onesactually measured, as recited in claim 23, the parameters can beadjusted, whereby a self-learning expert model can be accomplished.

The invention also relates to an arrangement as taught in claim 24.

The invention will now be explained in further detail by the descriptionthat follows, reference being made to the drawing, wherein

FIG. 1 shows a known table used in particular in Canada;

FIG. 2 shows a further known table as used ia in Europe;

FIG. 3 illustrates how HOT is estimated by the prior art;

FIG. 4 shows how, according to the invention, time intervals can begiven with great accuracy;

FIG. 5 schematically shows the principle of the invention;

FIG. 6 shows an example of the functioning of the calculation unit shownin FIG. 5;

FIG. 7 shows an embodiment of the invention combined with an expertsystem;

FIG. 8 shows the way in which the embodiment shown in FIG. 7 works;while

FIG. 9 shows how the invention can be combined with meteorologicalinformation for predicting the risk of ice, distributed over largedistances and periods of time.

FIG. 1 shows a table, Transport Canada, June 2002, used for estimationof HOT. The table is used “rearwards”, ie the pilot estimates visibilityin statute miles (the numbers given in the twelve cells). When thevisibility in snow is to be estimated it is of significance whether itis light or dark and moreover temperature plays a part as well. Thetable is used for estimating whether the snowfall is ‘heavy’, ‘moderate’or ‘light’. Then another table is used (not shown) that indicatesintervals for the equivalent amount of water in the precipitation as afunction of ‘heavy/moderate/light’, and once that is found it ispossible to resort to a further table (not shown) for obtaining a timeinterval for HOT as a function of the equivalent amount of water.

The method taught in the context of FIG. 1 is thus based on an estimate,ia of the visibility and how light or dark it is.

On 29 Jul. 2002 the National Center of Atmospheric Research published anarticle that explains the scientific reasons why visibility isunsuitable for use as decisive parameter as taught in connection withFIG. 1. It follows that this estimate is fairly uncertain.

FIG. 2 shows another known table indicating HOT time intervals if one isable to categorize the type of precipitation as one of the six types ofprecipitation shown in FIG. 2. The table can be used for varioustemperature intervals and for three different mixing ratios foranti-icing liquid.

FIG. 3 illustrates the method when the table shown in FIG. 2 is used.FIG. 3 is made to be, in principle, self-explanatory and hence only fewcomments will be made to FIG. 3. Particular attention is drawn to thefact that in FIG. 3 three estimates are made. In connection withinformation on the current weather being distributed every half hour(METAR), it is necessary, in case of sleet, to estimate whether it islight or heavy sleet. Then this first estimate is converted to anequivalent type of precipitation, being in the example light freezingrain. Use of the table as it is gives a holdover interval of 15-30minutes (corresponding to FIG. 2, top line under the column headed‘light freezing rain’). The upper limit of the time interval is thusgiven to be twice the lower limit, which is not reassuring. Besides thepilot has to perform the estimate that HOT is to be reduced if thevelocity of air or humidity of air is deemed to be high and, finally,the pilot has to estimate how the precipitation may change, if at all.

In practice this means that a pilot who drives for take off, typicallyin following wind conditions and with heavily slanted and heated windowpanes, is to be able to determine what the precipitation consists of(water, snow, sleet, super-cooled water, etc). In these conditions thepilot must later queue up for permission to take off and as time goes byperform an estimate whether the anti-icing liquid is still effective,having at his disposal only the very large margins of insecurity shownin FIG. 2; in adverse conditions the HOT may be as small as sevenminutes. Therefore accidents will occur when the table may indicate HOTto be as much as 30 minutes.

The known measurement equipment for measuring the composition of theprecipitation is able to measure drop size and estimate the distributionbetween snow and water, temperatures, dew point, etc., but is unable todistinguish whether a water drop is super-cooled or not, which iscrucial in the estimation of the risk of ice formation.

Reference is now made to FIG. 4 that shows a table like the one shown inFIG. 2, wherein, however, new measurement parameters are introduced, vizice factor and equivalent water amount.

It goes without saying that the equivalent amount of water in theprecipitation—in combination with the temperature to the left in thetable—is very significant to the amount of anti-icing liquid consumedduring a given period of time. Therefore it may be obvious to a personskilled in the art to introduce the equivalent liquid amount into thetable and receive information thereon through METAAR every half hour.This time interval is too large, but obviously it is an option totransmit the equivalent liquid amount more often when there is a risk ofice accumulation. However, the other issue is greater, and that is dueto the fact that the measurement methods used so far for finding theequivalent water amount has been associated with an inaccuracy of about30% around 0° C., where the problems solved by the invention are thegreatest. The measurement methods known so far have been unable todistinguish between how large a part of the deposited liquid drops aresuper-cooled and how large a part is not. According to the invention ameasurement of the actual amount of latent ice content in theprecipitation is measure, which is indicated by the numerals 1-9 in FIG.4. The dimension for the ice factor is the weight of deposited ice persurface unit per time unit.

By combining ice factor and equivalent amount of water in accordancewith the invention it is now possible in a reliable manner to providefar more accurate times/intervals for HOT, as will appear from FIG. 4,compared to what could be obtained by the prior art according to FIG. 2.

If for instance, the table is consulted under heading ‘snow’ and it isassumed that the ice factor is 2 and the equivalent water amount isbetween 0.4 and 0.88 mm it is possible to have fairly accurateinformation on the number of minutes for HOT. However, it is often thecase that the actual risk of ice formation (eg at ice factor 2) does notalways correlate with the equivalent water amount given in the table,viz 0.4 through 0.8. For instance, it is perceivable that the ice factoris measured to be 5, although the equivalent water amount is measured tobe within the range of from 0.4 to 0.8. This is due to the fact thatalmost all of the precipitation is super-cooled water, and therefore itis an option of one embodiment to select to enter the table under icefactor 5 and disregard the equivalent amount of water that was 0.4-0.8.

Alternatively the ice factor could be measured to be 1, the equivalentwater amount being, however, measured to be 0.4-0.8. Albeit the risk ofis form ation is in this case comparatively lower, there still remainsan amount of precipitation, eg snow under 0° C., that would consume alarger amount of anti-icing liquid than would be the case with an icefactor of 1. According to one embodiment the worst possible one of theice factor measurements or of the equivalent water amount measurement isselected as starting point for the calculation of HOT.

The embodiment just described for combination of ice factor andequivalent water amount is a simplified form of utilising the invention.On the basis of the explanations given above, it will be understoodthat, on the basis of tests, calculations and empirical tables, it willbe possible to weight the ice factor and the significance of theequivalent water amount, thereby ensuring that a sufficient amount ofanti-icing liquid is applied, while simultaneously use of redundantamounts of anti-icing liquid is avoided.

Table 4 includes some representative minute indicators for HOT. Suchvalues are not merely conditioned by calculations on the basis of saidmeasurements, but also on the safety requirements made by the airtraffic authorities.

It is noted that, in accordance with the invention, it is no longernecessary to read out a type of precipitation, and as such distinguishbetween the various types of precipitation, since the invention enablesa fairly accurate value for HOT. The reason why the types ofprecipitation are still included in FIG. 4 is the conservatism that willbe discussed at a later point in the specification, and that the typesof precipitation can now be determined even more accurately by means ofthe invention (which also correlates with the fact that HOT can bedetermined more accurately according to the invention).

FIG. 5 is a schematic view of an apparatus 1 known per se formeasurement of the density of liquid and frozen particles contained inthe precipitation and an apparatus 2 for measurement of the actualamount of ice deposited by the precipitation. According to theinvention, these measurement results are combined in a calculator unitthat is able to produce various output signals such as holdover time,HOT, composition of the precipitation and concentration of anti-icingliquid. The calculator unit receives other parameter values, too, suchas empirical values for HOT in response to the composition ofprecipitation, types of concentration of anti-icing liquid, etc.

As described above, the apparatus for measuring the actual amount of icein the precipitation could be determined eg by means of the apparatusknown from WO 00/540078 that is able to provide an exact result of howmuch is accumulated on a standard surface element erected on the airportpremises. Thereby it can be determined how much of the liquidprecipitation is super-cooled, but it cannot be deduced there from howquickly the anti-icing liquid will be consumed since the consumptiondepends on the type of precipitation, see the table shown in FIG. 2.Taking one's starting point in the empirical tables of holdover timethat are based on type of precipitation and combining that with anapparatus for measuring the actual ice formation, it is now possible todetermine the type of precipitation with great reliability and thereforethe large margins of insecurity of the known tables can be narrowed andin many cases replaced by a certain number of minutes for HOT when thetype of concentration of the anti-icing liquid is also entered as aparameter in the calculator unit shown in FIG. 3.

It is known that a certain degree of conservatism prevails within theaviation industry and it is therefore to be expected that a large numberof pilots would prefer to compare the objective and accurate holdovertime according to the invention to the teachings of the ‘old’ tables. Asmentioned and as taught by the invention an exact definition of the typeof precipitation is also accomplished and this can also be read out tothe pilot who is thereby able able to refer to the ‘old’ tables.

The very high degree of uncertainty that has so far been associated withthe prevention of accidents due to ice deposition has, of course,entailed an excess consumption of anti-icing liquid which is both veryexpensive and also a pollutant. By means of the accurate resultsaccomplished by the invention it is also possible to calculate‘backwards’, ie if as a starting parameter it is informed to thecalculator unit that one needs a holdover time of eg 35 minutes, thecalculator unit is able to produce an output signal that defines thetype and concentration of anti-icing liquid. FIG. 5 will show a fairlydetailed example of the calculation of the requisite type of anti-icingmixture, while simultaneously FIG. 5 illustrates how the results fromapparatus 1 and apparatus 2 are combined.

FIG. 5 explains which measurements are typically obtained by apparatus 1shown in FIG. 1. These measurements alone are associated with thedrawback that it is not possible to distinguish between rain andsuper-cooled rain, but by combining the measurements from apparatus 1with measurements from an apparatus 2 in accordance with the invention,it is possible to determine the actual amount of ice accumulated,whereby a far more reliable estimate of HOT is accomplished compared towhat was possible with the prior art. It should be noted that thedesignations ‘apparatus 1’ and ‘apparatus 2’ need not necessarily be twophysically different apparatuses; rather they express the measurementprinciples applied and explained in cells 11 and 12, respectively, inFIG. 5. It also applies that in case of physical movement apparatus 2 isunable to distinguish between heavy precipitation of very fine snow withsmall adhesive capability and light precipitation in the form of wetsnow with correspondingly large adhesive capabilities; this difference,however, can easily be determined by apparatus 1 on the basis of thedifference in reflectivity. The two measurement principles 1 and 2therefore supplement each other in a particularly advantageous mannerfor achieving a reliable determination of the type of precipitation, seecell 13. According to a preferred embodiment, the measurements inapparatus 2 are performed at different rates of rotation for themeasurement element in order to further enhance measurement reliability.As it is, an immediate combination of the measurements made byapparatuses 1 and 2 in case of a typical movement of the measurementelement does not enable distinction between sleet (water/snow) and otherwater and ice-particle mixtures (eg water/hail). This distinction can beobtained by performing measurements in apparatus 2 at a number ofdifferent rates of rotation, whereby the different whirl-offcharacteristics of various ice particles and water can be taken intoaccount. The latter measurements are illustrated in cell 14, such thatin cell 15 an even more reliable determination is obtained of the natureof the precipitation. This was what was the major problem of the priorart when eg the table shown in Figure 2 was to be used. When thereliable determination of type of precipitation is combined with theexperience numbers shown in cells 16 and 17 it is possible to obtain avery reliable determination of HOT in cell 18. This very reliabledetermination makes it possible in practice, too, to “calculaterearwards”, ie when the very accurate HOT is known in cell 18, it ispossible by combination with the desired durability (cell 19) to deducethat if it is desired to have a durability of 12 minutes an anti-icingliquid should be in a mixture ratio of 88% of anti-icing liquid and 12%of water, see cell 20.

It is noted that the values given in connection with the figures serveas examples only, as a complete set of values is very comprehensive andin practice something that is defined in cooperation with the airtraffic authorities.

FIG. 6 shows some further advantages of the invention. The calculatorunit mentioned in FIG. 3 is now a constituent of a larger computer thatcomprises a mathematical model for estimating an expected result. Theexpected result was compared in the computer to subsequent actualmeasurements of the actual amount of ice formed and in case of adeviation an adjustment is automatically performed of the parameters ofthe mathematical model that will, in this manner, become a self-learningexpert model. The prerequisite for this to be accomplished is preciselythe combination according to the invention: viz that a number ofreliable measurement results are provided that are entered into themodel and that exact knowledge is provided about the ‘true’ result usedfor performing automatic adjustments of the mathematical model. It willbe understood that the apparatus for measurement of the actual amount ofice can be elaborated on in a variety of ways. For instance it ispossible to perform particular procedures on various sequences ofrotation and temperature in the apparatus and subsequent measurement ofaccumulated amount of ice, ice structure, measurement of air resistance,density of the ice, and it is also an option to spray the surfaceelement with anti-icing liquid. In practice, not all of suchmeasurements can be performed immediately before each and every plane isdue for application of anti-icing liquid, when the traffic is dense, butthese measurements can, when traffic is less dense, contribute tooptimisation of the mathematical model, thereby considerably increasingthe reliability of the rather short-termed measurements that areperformed immediately before anti-icing liquid is applied to a plane.

FIG. 7 illustrates how the invention can be used in combination with anexpert system. The functions in cells 21-24 are immediately recognizablein view of the above explanations. Thus cell 25 contains the resultsthat can be achieved by means of the combination unit shown in FIG. 4 aswas explained in the context of FIG. 5. Cell 26 contains information onthe one hand on the most recently calculated result and previouslycalculated results and when this is compared to the absolutely mostrecent result the parameters of an expert model can be optimized suchthat both yet more well-defined results are obtained in cell 27 and theoption is provided of projecting a result eg 20 minutes ahead in time,see cell 28.

In principle, FIG. 8 shows how a global network of information relatingto the risk of icing can be built. According to the invention thecalculations can be supplemented with meteorological information. FIG. 8schematically shows fronts of depressions on their way across the Northsea and the computer in one of London's airports 31 now containing exactknowledge of the risk of icing and how it was distributed during passageof the fronts. This information and the meteorological information canbe used first in Billund 32 and then in Copenhagen 33 where theadjustments that subsequently appear at the passage of the front can beused at the airport in Stockholm 34 when the fronts pass.

The described considerable improvements in air-traffic safety could nothave been obtained by means of a known expert system in combination withthe empirical and very uncertain determinations of the risk of icingknown so far. The high degree of inaccuracy achieved by the inventionenables use of advanced calculation models to impart reliable valued atthe individual airports and such that the reliability can be furtherenhanced by means of measurements performed in other airports.

1. A method of determining the risk of ice deposition due toprecipitation, wherein air temperature is measured and a type ofprecipitation and an amount of precipitation are estimated,characterised in that a measurement is performed for determining theactual amount of ice contained in the precipitation; and that theresults from said measurements are combined for determining the risk ofice deposition.
 2. A method according to claim 1, characterised in thatthe type of precipitation is estimated on the basis of a measurement fordetermining the ratio of liquid to frozen particles contained in theprecipitation.
 3. A method according to claim 1, characterised in thatthe measurement for determining the ratio of liquid to frozen particlesis determined by an optical measurement known per se and subsequentcalculation.
 4. A method according to claim 1, characterised in that ameasurement is performed for determining the total equivalent, liquidamount of precipitation.
 5. A method according to claim 1, characterisedin that the measurement for determining the actual amount of icecontained in the precipitation is performed as a calculation on thebasis of dew point measurement.
 6. A method according to claim 1 or 5,characterised in that the measurement for determining the actual amountof ice contained in the precipitation is performed as a measurement ofactual ice formation.
 7. A method according to claim 6, characterised inthat the measurement comprises provision of a surface element that has apredetermined surface area and is, during a predetermined period oftime, caused to move relative to the atmospheric air, following whichthe amount of ice accumulated on the surface element during said periodof time is measured.
 8. A method according to claim 6, characterised inthat the temperature of the surface element is caused to correspondessentially to the temperature of the atmosphere.
 9. A method accordingto claim 6, characterised in that the temperature of the surface iscaused to have another predetermined temperature during said period oftime.
 10. A method according to any one of claims 7-9, characterised inthat, following measurement of the accumulated amount of ice, a relativemovement is briefly provided between the surface element and theatmosphere at a rate that considerably exceeds the rate prior to saidmeasurement, following which a further measurement of deposited ice isperformed.
 11. A method according to any one of claims 7-10,characterised in that the relative rate between the surface element andthe atmosphere is controlled by controlling the rate of rotation of oneor more rotatable surface elements.
 12. A method according to claim 11,characterised in that the adhesive capacity of the ice is measured bymeasurement of the accumulated amount of ice following a number ofrotations at mutually different rates.
 13. A method according to claim11, characterised in that the air resistance between the atmosphere andthe ice accumulated on the surface element is measured.
 14. A methodaccording to any one of claims 7-13, characterised in that anti-icingliquid is applied in a predetermined concentration and a predeterminedamount on the surface element before the measurements are performed. 15.A method according to any one of claims 1-14, and wherein a surface isapplied with a predetermined type and concentration of anti-icingliquid, characterised in that the risk of ice deposition on the surfaceis calculated on the basis of knowledge of the type and concentration ofthe anti-icing liquid; knowledge of the result of the measurement fordetermining the ratio of liquid to solid particles contained in theprecipitation; and knowledge of the result of the measurement fordetermining the current amount of ice contained in the precipitation.16. A method according to claim 15, characterised in that the risk iscalculated and shown as a holdover time.
 17. A method according to claim15 and wherein manual tables are used to estimate holdover time, whereinthe tables are grouped according to different types of precipitation,characterised in that the knowledge of the current amount of icecontained in the precipitation and the ratio of the precipitation ofsolid to liquid particles is used for defining the type ofprecipitation.
 18. A method according to claim 15, characterised in thatthe concentration and anti-icing liquid is determined as a function ofholdover time and the measured risk of ice deposition.
 19. A methodaccording to any one of claims 1-18, characterised in that an expertsystem is used for the calculations which is configured for being run ona computer and configured for being able to estimate the risk of icedeposition on the basis of measurements, and configured for receivinginformation about the actual amount of ice accumulated, and, on thebasis of the difference between the calculated and actual amount of ice,adjusting parameters in a calculation model for calculating thedeposited amount of ice.
 20. A method according to claim 19,characterised in that the computer is caused to be in communicativeconnection with other computers that are located in geographicallydifferent places; and that the expert system is configured forcalculating in advance future changes with regard to the risk of icedeposition in response to meteorological parameters entered therein. 21.An apparatus for exercising the method according to claim 1,characterised in that the apparatus comprises a combination of opticalmeans for measuring the reflectivity of precipitation; and mechanicalmeans for moving a measuring surface element in relation to the air andfor measuring the amount of ice accumulated on the surface elementduring a given period of time; and electronic means for combining saidmeasurements.
 22. An apparatus according to claim 21 and for thecalculation of holdover time for anti-icing liquid, characterised inthat it comprises a data storage for storing information about empiricalvalues for holdover time as a function of type of precipitation and theconcentration of the anti-icing liquid.
 23. An apparatus according toclaim 21 or 22, characterised in that it comprises a mathematical modelfor estimating the ice deposition due to precipitation; and that theelectronic means are configured for comparing the estimated values tothe actually measured values for the amount of ice and for adjustingparameters in the model for optimisation thereof.
 24. An arrangement forpredicting the risk of ice deposition due to precipitation,characterised in that it comprises a number of apparatuses of the kinddescribed in claims 21-23 that are arranged on different geographicallocations and are configured for receiving meteorological informationabout the movement of air substances